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<html><head><title>SRFI 87: =&gt; in case clauses</title></head><body>

<h1>Title</h1>

=&gt; in case clauses

<h1>Author</h1>

Chongkai Zhu

<h1>Status</h1>

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<p>
</p><ul>
  <li>Received: <a href="http://srfi.schemers.org/cgi-bin/viewcvs.cgi/*checkout*/srfi/srfi-87/srfi-87.html?rev=1.1">2006/04/10</a></li>
  <li>Revised: <a href="http://srfi.schemers.org/cgi-bin/viewcvs.cgi/*checkout*/srfi/srfi-87/srfi-87.html?rev=1.2">2006/05/08</a></li>
  <li>Draft: 2006/04/10 - 2006/06/08</li>
  <li>Final: <a href="http://srfi.schemers.org/cgi-bin/viewcvs.cgi/*checkout*/srfi/srfi-87/srfi-87.html?rev=1.3">2006/10/18</a></li>
</ul>

<h1>Abstract</h1>This SRFI proposes an extension to the <code>case</code> syntax 
to allow the <code>=&gt;</code> clauses as in <code>cond</code>.
<h1>Rationale</h1><code>case</code> is introduced as a syntax sugar based on 
<code>cond</code>, which helps to save a explicit calling to <code>let</code>. 
But without the <code>=&gt;</code> clause, if the <code>result </code>expression 
needs the value of <code>key</code>, the <code>let </code>can't be saved. For an 
easy example, suppose we want the following:
<blockquote><pre>(case (get-symbol)
  ((true) #t)
  ((false) #f)
  (else =&gt; (lambda (x) x)))</pre></blockquote>
<p>Without the <code>=&gt;</code> clause in <code>case</code>, we have to 
write:</p>
<blockquote><pre>(let ((key (get-symbol)))
  (cond ((eq? key 'true) #t)
        ((eq? key 'false) #f)
        (else key)))</pre></blockquote>
<h1>Specification</h1>
<p>(Based on R5RS section 4.2.1 Conditionals)</p>
<dl>
  <dt><u>library syntax:</u> <b><a name="case">case</a></b> <i>&lt;key&gt; &lt;clause1&gt; 
  &lt;clause2&gt; ...</i> 
  </dt><dd><em>Syntax:</em> &lt;Key&gt; may be any expression. Each &lt;clause&gt; 
  should have the form <pre><tt>((&lt;datum1&gt; ...) &lt;expression1&gt; &lt;expression2&gt; ...),</tt></pre>
  <p>where each &lt;datum&gt; is an external representation of some object. All 
  the &lt;datum&gt;s must be distinct. The last &lt;clause&gt; may be an "else 
  clause," which has the form </p><pre><tt>(else &lt;expression1&gt; &lt;expression2&gt; ...).</tt></pre>
  <p><font color="#ff0000">Alternatively, a &lt;clause&gt; may be of the form 
  </font></p><pre><font color="#ff0000"><tt>((&lt;datum1&gt; ...) =&gt; &lt;expression&gt;)</tt></font></pre>
  <p><font color="#ff0000">and the last &lt;clause&gt; may be of the 
  form</font></p><pre><font color="#ff0000"><tt>(else =&gt; &lt;expression&gt;)</tt></font></pre>
  <p><em>Semantics:</em> A <samp>`case'</samp> expression is evaluated as 
  follows. &lt;Key&gt; is evaluated and its result is compared against each 
  &lt;datum&gt;. If the result of evaluating &lt;key&gt; is equivalent (in the 
  sense of <samp>`eqv?'</samp>; see section see section <a href="http://www.swiss.ai.mit.edu/%7Ejaffer/r5rs_8.html#SEC49">6.1 Equivalence 
  predicates</a>) to a &lt;datum&gt;, then the expressions in the corresponding 
  &lt;clause&gt; are evaluated from left to right and the result(s) of the last 
  expression in the &lt;clause&gt; is(are) returned as the result(s) of the 
  <samp>`case'</samp> expression. If the result of evaluating &lt;key&gt; is 
  different from every &lt;datum&gt;, then if there is an else clause its 
  expressions are evaluated and the result(s) of the last is(are) the result(s) 
  of the <samp>`case'</samp> expression; otherwise the result of the 
  <samp>`case'</samp> expression is unspecified. <font color="#ff0000">If the 
  selected &lt;clause&gt; uses the <code>=&gt;</code> alternate form, then the 
  &lt;expression&gt; is evaluated. Its value must be a procedure that accepts 
  one argument; this procedure is then called on the value of &lt;Key&gt; and 
  the value(s) returned by this procedure is(are) returned by the 
  <samp>`case'</samp> expression. </font></p></dd></dl>
<p>(Based on R5RS section 3.5 Proper tail recursion)</p>
<dd>If a <tt>cond</tt> <font color="#ff0000">or <tt>case</tt></font> expression is in a tail context, and has a clause of the 
form <tt>(&lt;expression<sub>1</sub>&gt; =&gt; &lt;expression<sub>2</sub>&gt;)</tt> then the 
(implied) call to the procedure that results from the evaluation of &lt;expression<sub>2</sub>&gt; 
is in a tail context. &lt;expression<sub>2</sub>&gt; itself is not in a tail context.</dd>
<h1>Implementation</h1>
<blockquote><pre><code>(define-syntax case
  (syntax-rules (else =&gt;)
    ((case (key ...)
       clauses ...)
     (let ((atom-key (key ...)))
       (case atom-key clauses ...)))
    ((case key
       (else =&gt; result))
     (result key))
    ((case key
       ((atoms ...) =&gt; result))
     (if (memv key '(atoms ...))
         (result key)))
    ((case key
       ((atoms ...) =&gt; result)
       clause clauses ...)
     (if (memv key '(atoms ...))
         (result key)
         (case key clause clauses ...)))
    ((case key
       (else result1 result2 ...))
     (begin result1 result2 ...))
    ((case key
       ((atoms ...) result1 result2 ...))
     (if (memv key '(atoms ...))
         (begin result1 result2 ...)))
    ((case key
       ((atoms ...) result1 result2 ...)
       clause clauses ...)
     (if (memv key '(atoms ...))
         (begin result1 result2 ...)
         (case key clause clauses ...)))))</code></pre></blockquote>
<h1>Copyright</h1>Copyright (C) 2006 Chongkai Zhu. All Rights Reserved. 
<p>Permission is hereby granted, free of charge, to any person obtaining a copy 
of this software and associated documentation files (the "Software"), to deal in 
the Software without restriction, including without limitation the rights to 
use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of 
the Software, and to permit persons to whom the Software is furnished to do so, 
subject to the following conditions: 
</p><p>The above copyright notice and this permission notice shall be included in 
all copies or substantial portions of the Software. 
</p><p>THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR 
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS 
FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR 
COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER 
IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN 
CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE. 
</p><hr>

<address>Editor: <a href="mailto:srfi-editors@srfi.schemers.org">Mike 
Sperber</a></address><!-- Created: Tue Sep 29 19:20:08 EDT 1998 --><!-- hhmts start -->Last 
modified: Mon Apr 10 21:20:25 CEST 2006 <!-- hhmts end -->
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